This forum is about wrong numbers in science, politics and the media. It respects good science and good English.
There's a lot of it you know, something like three times the mass of petrol used. Drive thirty miles and you've got 6-10 kg of the stuff to get rid of. No way that could be stored in a small amount of ceramic or anything. Drive to your 300 mile refuelling range and the amount weighs as much as a passenger.
(all amounts correct only to order of magnitude!)
It seems ever clearer to me that the 'greenies' collectively, no matter which wing of greenism they stand for, are all saying the one trunk or four legs good (unless it's a industrial farm animal) and two legs bad (unless it also has wings or a tail.)
So the key is to reduce the population of the world. To nothing if possible if some of them are to be taken at their word.
Now it occurs to me that they believe that Humanity is doing just that already so I suppose their main concern must be for the plants and the creatures (4 or more legs or with wings or tails) that rely upon them. Otherwise they would be encouraging all those things they discourage since they must believe they will create ever faster changes leading to the destruction of the planet as it currently exists.
On the other hand one could suggest that the only purpose of humankind is to correct the damaging loss of plant life giving carbon when so much of it was sequestered, presumably accidentally, at some point before recorded history began.
Clearly humanity was created specifically to correct this accident and, having taken a while to work out how, may now be in a position to deliver a complete solution and return the planet to the verdancy and temperatures it must have enjoyed when the dinosaurs were around.
I can't imagine why tree lovers would wish to eliminate such activity. Especially since once the CO2 has been liberated there would be no need for humanity ust at the point where humanity might well have no viability. Perfect synergy!
As for hydrogen - a touch of the vapours surely? Would there really be no effect on anything from the H2O exiting the tailpipe?
(going mad just like all the others, but possibly in a different direction ...)
As water vapour is, in general, not a well mixed "gas" a pure hydrogen fuel will bump up the levels of water vapour in the air quite nicely in populated and urban areas. Urban heat island effect will get worse. Water vapour is a more effective greenhouse gas than carbon dioxide and pure hydrogen is a less effective fuel than liquid hydrocarbon there will be more greenhouse effect emissions per horsepower produced.
Another fine example of the law of unintended consequences.
Now if a system for extracting hydrogen from water and carbon from atmospheric carbon dioxide to make liquid hydrocarbon fuel could be developed everything pretty much stabilises. Carbon from exhaust carbon dioxide released the atmosphere is recycled into fuel, water vapour from the exhaust recycles as rain in the usual way. Hence any variation is simply linked to number of hydrocarbon engines in use, their mean efficiency, mean power output and mean run time. There is no continuous accumulation.
And if you want to reduce atmospheric carbon dioxide extending the process to produce feed-stock for plastic materials might actually provide some sort of economic return. Have more chance than pumping the stuff into caves.
Of course this sort of thing will increase the oxygen content of the atmosphere but, as far as les greens are concerned, oxygen is perfectly safe and a bit more makes no difference.